1. Field of the Invention
This invention relates to a portable device for use in the measurement of a component content of a material, particularly the water content of a material and water and fat content of whole fish, using the dielectric properties of the component. The rapid determination of water and fat content of products is a prevailing problem in the fish industry, since these contents affect various processing operations of the fish, for example, when smoking the fish. In this respect, manufacturers need to determine the fat content of fish so as to assess final product quality--a medium fat content indicates a better quality than either low or high fat content. For similar reasons fish farmers need to assess the fat content of live fish before selling to customers.
2. Discussion of Prior Art
In whole fish flesh, such as herring and other pelagic species, the problem is to a certain extent simplified by the fact that a linear relationship exists between the fraction of water and that of fat (Iles and Wood, 1965) so that the measurement of one serves to determine the other within the limit of this relationship. Such a relationship arises from the physiological need of the fish to maintain a slightly negative buoyancy even while the fat content, which is of low density, tends to creat the opposite effect. Variation in fat content arises from seasonal effects of food availability and breeding cycle.
Tests do exist for the measurement of water and fat content of fish but the main disadvantage of these tests is that they are destructive in that the fat must firstly be extracted from the fish, then after mixing this with a solvent, chemical analysis is used to determine the fat and water contents. These current methods also have the added disadvantage of being very time-consuming, and cannot be used on whole or live fish. Nuclear Magnetic Resonance (NMR) may also be used but is expensive and bulky.
It is known (Mudgett et al (1974), J Food Sci, 39, 632-635) that in the microwave frequency region the dielectric properties of materials are dominated by those of water especially if the water fraction is large. Thus the attenuation of microwave energy by a material provides a measure of the moisture content of the material. It is also known (Ohlsson et al (1974) J Food Sci, 39, 1153-1156) that both the fat and water contents of meat emulsions could be determined from various dielectric measurements in the microwave range. However, in those data no correlation existed between the fat and water content.
It is further known (Kent 1989) that there is a good correlation between the water content of the fish and the real part of the dielectric permittivity at all frequencies in the microwave range. There exists many methods for the measurement of the dielectric properties of materials, one such method being the subject of UK Patent No 1354474. This method depends on the dielectric sensitive properties of a form of transmission line known as a microstrip or stripline sensor.
However, those known methods of the measurement of the dielectric properties of materials all use apparatus which are large and bulky, inconvenient and not portable.
There is therefore a need for a portable device for the measurement of the dielectric properties of materials, in particular the water content of materials and the fat and water content of whole or live fish which has the advantages of being both convenient and easy to use.